Page 390 - Advanced Mine Ventilation

Spontaneous Combustion of Coal 359

In United States, slightly more than 50% of underground coal mining is done by the
retreat longwall method. Main benefits of longwall mining are as follows:
• Improved safety and reduced injury rates because of improved longwall-to-development coal
ratios and fewer longwall moves.
• Improved recovery of in situ coal.
• Improved productivity and reduced cost per ton.

21.3.2.1 Pillar Extraction by Continuous Miners
This technique is a poor choice to control heatings. There are many variations of this
technique but, generally speaking, the coal pillars are further subdivided by the contin-
uous miners, using “split and lift” technique. A lot of coal is left in the gob and small
remnant pillars are exposed to increased abutment pressures resulting in pillar failures.
If heating occurs, it is difficult to seal the area. Most often, the entire panel has to be
sealed.

21.3.2.2 Longwall Mining

Longwall blocks of coal are commonly developed to have a width of 700e1500 feet
and a length of 10,000e15,000 feet. To get enough air to ventilate, usually a three-
entry development is used in United States. Chain pillars are as large as possible
with minimum crosscuts. This is by far the safest method of mining coal especially
when it is prone to spontaneous combustion. Longwall mining of low-rank coal, liable
to spontaneous heating require that no bleeders/bleeder shafts are used; the newly
created gob is progressively sealed and in most cases the gob is inerted with nitrogen.
These subjects will be discussed next. In summary, general guidelines for safe second-
ary recovery are as follows:

1. Minimize coal lost in the gob.
2. Maximize consolidation of gob areas.
3. Maintain a steady mining rate.
4. Do not use bleeder ventilation fan.
5. Design the extraction panels for quick sealing.
6. Make sure the gob areas are monitored and kept inert, that is, oxygen at less than 4%,
preferably 2%.

21.3.3 Mine Ventilation and Methane Control on Longwall
Faces
Earlier discussed mine ventilation and methane control techniques need modifications
if the coal seam is liable to spontaneous combustion. Main changes are listed below:
1. The district (number of panels to be mined contiguously) must be limited to 3e4 longwall
panels so it can be completely sealed in 3e4 years.
2. The longwall panels should be modest in size (1000 ft 10,000 ft) so it can be mined in less
than a year.